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The present invention relates to coaxial cables, and more particularly to coaxial cables having improved mechanical and electrical properties.
A typical coaxial cable used for data, voice and signal transmission comprises a central conductor surrounded by an insulating foam layer, a screen layer surrounding the insulating foam layer and a protective exterior jacket surrounding the screen layer. Other typical coaxial cable designs include two additional insulation layers of solid material surrounding one of them the central conductor and the other the insulating foam layer.
The insulating structure, which surrounds the central conductor, has several functions such as to separate the central conductor from the screen layer, while keeping the dielectric losses to a minimum. Air is known as one of the best insulators available, and the foam is typically made from a foamed compound having a high content of air bubbles which serves as an excellent insulator, therefore, the more air bubbles the foam layer have the better insulating properties will have thereof.
The dielectric and mechanical characteristics of the coaxial cables are of a great importance in order to assure an optimum data, voice and signal transmission and to avoid losses or distortion of data. It has been observed that when the coaxial cable is severely manipulated, the material of the foam layer tends to compress or to bend due to mechanical stresses, which deforms the foam layer, and consequently causes variations of the dielectric properties, leading to distortion of data, data losses, etc. It should be noted that in drop coaxial cables the adherence between the conductor and the insulation structure is extremely important since poor adherence between these two components may cause installation or connection problems and moisture can penetrate during service into the eke deteriorating its dielectric properties.
Therefore, it would be highly desirable to have a coaxial cable having high adherence between the central conductor and the insulating foam layer to protect it against mechanical stress and moisture penetration. This high adherence can preserve the mechanical and electrical properties of the coaxial cable during severe manipulation. One solution to this problem is to apply adhesives between the conductor and the insulation structure, such as those disclosed in the U.S. Pat, Nos. 2,970,129, 3,520,861, 3,681,515 and 3,795,540. However, such adhesives adversely affect the electrical properties of coaxial cables. Another solution to protect the foam layer against mechanical stress and moisture is by having multilayer insulating structures. Structures disclosed by Nishikawa (U.S. Pat. No. 6,239,377) and Hvizd (U.S. Pat. No. 3,287,489) have a three-insulation layer configuration. However the disclosed functions of the multilayer insulation structures in these patents do not disclose the high adhesion levels required between the foam layer and the central conductor in order to protect the foam layer against mechanical stress without adversely affecting the electrical performance of the coaxial cable.
In view of the above referred problems, intensive experimental work was undertaken to increase the adhesion between the central conductor and the foam insulation layer without adversely affecting the electrical properties of coaxial cables. Surprisingly, it was discovered that blending a polyolefin with an ionic hydrocarbon polymer within a narrow concentration range and applying it between the inner conductor and the foam insulation layer produced adhesion strengths higher than 40 lb/in2 without using any adhesive and without affecting any electrical property of the coaxial cable. However, field experience indicates that adherence values higher than 60 lb/in2 cause installation problems due to the extra effort required stripping the insulation. Another problem is that residues are left on the central conductor, which must be removed in order to have good electrical contact. Furthermore, it was found that the Return Loss and Structural Return Loss were surprisingly improved.
When a coaxial cable is operating, a signal traveling down the line is reflected partially and the reflection travels back to the signal source. These reflections are caused by variations in conductor diameter, diameter, degree of foaming and dielectric constant of the insulation layer, eccentricity, and surface imperfections between layers. Return Loss (RL) is the electrical measurement to quantify the variation in the characteristic impedance along the frequency spectrum. SRL (structural return loss) describes the portion of the return loss, which is due to structural changes along the cable.
It is accordingly a major object of the present invention to provide a coaxial cable having an insulating multilayer structure having improved tolerance to severe manipulation during its installation, connection and useful service life. It is still a main objective of the present invention to provide a coaxial cable with improved data, voice and signal transmission characteristics resulting from enhanced Return Loss and Structural Return Loss properties.
It is another object of the present invention to provide a coaxial cable having a solid polyolefin inner layer with an adherence between 40 and 60 lb/in2 between the central conductor and the inner solid insulating layer; an intermediate foamed polyolefin layer surrounding the inner layer; an outer solid polyolefin layer surrounding the intermediate foamed layer; a metallic screen layer surrounding the outer polyolefin layer and a protective exterior jacket surrounding the screen layer.
Another object of the present invention is to provide a coaxial cable wherein the solid polyolefin inner insulation layer comprises a blend of low-density polyethylene with 8 to 15 weight percent of an ionic hydrocarbon polymer. The ionic hydrocarbon polymer of the present invention contains monocarboxylic acid comonomers, wherein the carboxylic acid units of the copolymers are partially neutralized with metal ions. Such materials are sold by E. I. du Pont de Nemours and Company under the trade name SURLYN(trademark) as disclosed in the U.S. Pat. No. 3,264,272.
Another object of the present invention is to provide a coaxial cable with a minimum of 36 dB of Return Loss and 39 dB of Structural Return Loss in the frequency range from 500 to 1000 MHz as a result of using an inner solid insulating layer consisting of a blend of 8 to 15 weight percent of an ionic hydrocarbon polymer